1. Field of the Invention
The present invention relates to a switching device driving apparatus that drives a switching device on receiving a control signal, and particularly relates to a switching device driving apparatus that performs synchronous rectification control of a first and second switching devices connected in series included in a switching apparatus.
2. Description of the Related Art
An inverter or the like for controlling a motor performs drive control of a motor by controlling a switching apparatus to switch current supply paths to coils of the motor. A transformer or the like transforms a voltage generated by a power supply into any desired voltage by controlling a switching apparatus to adjust the current supply amount from the power supply to a reactor (coil).
A switching apparatus includes a first switching device and a second switching device connected in series and provides current from the connecting point of the first and second switching devices as an output unit to a reactor connected thereto. In order to drive the first and second switching devices of the switching apparatus, a switching device driving apparatus is required.
One switching device driving apparatus of this type is a switching device driving apparatus that performs synchronous rectification control of the first switching device and the second switching device. Synchronous rectification control is a control method in which, when the first switching device is ON, the second switching device is caused to be OFF, and when the first switching device is OFF, the second switching device is caused to be ON. For this control method, an invention has been proposed that provides a delay (dead time) in order to avoid overlapping of the conducting period (hereinafter referred to as ON period) of the second switching device with the ON period of the first switching device (in other words, in order to avoid what is called arm short circuit).
For example, JP-A-2002-335679 discloses a switching device driving apparatus that includes a circuit for splitting a common control signal into two and inverting one of them and a delay circuit for delaying the rising or falling of the control signal, and performs synchronous rectification control of a first switching device and a second switching device. This switching device driving apparatus provides a dead time to avoid arm short circuit by delaying the rising or falling of the control signal.
Furthermore, for example, JP-A-2009-290812 discloses a switching device driving apparatus that includes a circuit for detecting a edge of output signals of a first switching device and a second switching device, a circuit for adjusting the voltage of the detected edges, a comparative circuit for comparing the adjusted edges to determine a delay amount and a circuit for providing a delay to a control signal depending on the delay amount, and performs synchronous rectification control of a first switching device and a second switching device. This switching device driving apparatus provides an amount of delay depending on the detected edges to avoid arm short circuit.
For the switching device driving apparatus disclosed in the JP-A-2002-335679, when the delay is provided by a capacitor, the capacitance may vary due to temperature change, which may not ensure a desired dead time, leading to arm short circuit.
For the switching device driving apparatus disclosed in the JP-A-2009-290812, since a time taken to detect the edge is used as a given dead time, a desired dead time may not be ensured at the start-up depending on ambient temperature, leading to arm short circuit.
Also, even when the rising timing of a signal output from the switching device driving apparatus for driving a switching device is delayed or the rising speed of that signal is decreased with respect to the rising of a signal input to the switching device driving apparatus for controlling the switching device, variation in the devices and the delay of the signal for driving the switching device with respect to the signal for controlling the switching device (output propagation delay) may increase when ambient temperature is high, which may not ensure a desired dead time, leading to arm short circuit.
The shorter the dead time is, the more efficient the switching apparatus is. But, when the desired dead time is not ensured, causing arm short circuit, the switching device will be burnt and, at worst, a fire will occur. Also, since synchronous rectification is often used for a DC-DC converter for transforming a voltage generated by an electric generator into a voltage suitable for an equipment connected thereto or often used for an inverter for controlling a motor, the arm short circuit may cause the power supply or the motor to be stopped, even though a fire will not occur.